For digital microwave communication, a dual polarization transmission method is used to use frequency more efficiently: According to the dual polarization transmission method, orthogonal polarizations, a vertical polarization (referred to as V-polarization, hereinafter) and a horizontal polarization (referred to as H-polarization, hereinafter), of a radio wave are transmitted at the same frequency. According to the above method, the use of the same frequency for the vertical polarization and horizontal polarization, as well as the orthogonality of an antenna or space, is not perfect. Therefore, both polarizations leak into each other. The leakage is called cross polarization interference, having a negative effect on the transmission quality of signals. In particular, when the dual polarization transmission method is used together with a multiple-valued modulation/demodulation method such as QAM (Quadrature Amplitude Modulation), the effect is significant.
Accordingly, interference components are removed by a cross polarization interference compensator (XPIC: Cross Polarization Interference Canceller). A polarization that the XPIC handles is defined as one polarization, and a polarization crossing one polarization at right angles as the other polarization. To compensate for interference from the other polarization, synchronization of the local oscillation (local) frequencies of one polarization and the other polarization is important.
What is disclosed in PTL1 is about a reception local synchronous method as a local synchronous method. There are two reception local synchronous methods: a common local method, in which one local oscillator is shared by a one-polarization receiver and an other-polarization receiver; and a reference synchronous scheme, in which a one-polarization receiver and an other-polarization receiver each have separate local oscillators, with a reference signal (Reference Signal) of each local oscillator shared.
According to the common local method, the configuration of a circuit is simple. However, if the local oscillator breaks down, signals of both polarizations could be turned off at the same time. Conversely, according to the reference synchronous scheme, the configuration of a circuit is complex. However, if a local oscillator breaks, the effects can be limited to either one polarization or the other polarization. Therefore, in many cases, the system is highly redundant with the use of the reference synchronous scheme in order to improve the efficiency in using radio waves.
FIG. 3 is a block diagram showing the configuration of a transmitting/receiving system of a reception local synchronous type with the use of the reference synchronous scheme, which is a related technique.
In the case of FIG. 3, the following components are provided at a transmitting side: modulators (V/H modulators) 1 and 2 for V- and H-polarizations; transmitters (V/H transmitters) 11 and 12 for V- and H-polarizations; transmitter local oscillators (V/H transmitter local oscillators) 15 and 16 for V- and H-polarizations; and transmitting antennas (V/H transmitting antennas) 21 and 22 for V- and H-polarizations. The V/H transmitters 11 and 12 respectively include front stage-side amplifiers 111 and 121, mixers (multipliers) 112 and 122, subsequent stage-side amplifiers (power amplifiers) 113 and 123. The V/H transmitter local oscillators 15 and 16 respectively include PLL (Phase Locked Loop) circuits 152 and 162 having VCOs (Voltage Controlled Oscillators) 151 and 161.
The following components are provided at a receiving side: receiving antennas (V/H receiving antennas) 23 and 24 for V-/H-polarizations; receivers (V/H receivers) 13 and 14 for V-/H-polarizations; receiver local oscillators (V/H receiver local oscillators) 17 and 18 for V-/H-polarizations; and demodulators (V/H demodulators) 3 and 4 for V-/H-polarizations. On the receiving side, a reference circuit 30 is also provided that outputs a common reference signal to the V/H receiver local oscillators 17 and 18. The V/H receivers 13 and 14 respectively include front stage-side amplifiers (low noise amplifiers) 131 and 141, mixers 132 and 142, and subsequent stage-side amplifiers 133 and 143. The V/H receiver local oscillators 17 and 18 respectively include PLL circuits 172 and 182 having VCOs 171 and 181. The V/H demodulators 3 and 4 each include a XPIC (not shown) that accepts a demodulation signal of the other polarization-side V/H demodulators 3 and 4 and obtains a correlation between the demodulation signal and an error signal obtained from one polarization to eliminate cross polarization interference components.
According to the above configuration, at the transmitting side, signals of V- and H-polarizations are modulated from baseband signals to IF (Intermediate Frequency) signals in the V/H modulators 1 and 2. After the modulation, the IF signals pass through the V/H transmitters 11 and 12, respectively, before being multiplied by outputs of the V/H transmitter local oscillators 15 and 16. In this manner, frequency conversion takes place from IF signals to RF (Radio Frequency) signals. After the conversion, the RF signals are transmitted as radio waves of V- and H-polarizations from the V/H transmitting antennas 21 and 22, respectively.
At the receiving side, the V/H receiving antennas 23 and 24 receive radio waves of the V- and H-polarizations, respectively, which have travelled through a space from the transmitting side. The received radio waves of V- and H-polarizations respectively pass through the receivers 13 and 14 from the V/H receiving antennas 23 and 24 before being multiplied by outputs of the V/H receiver local oscillators 17 and 18. In this manner, frequency conversion takes place from RF signals to IF signals. After the conversion, the IF signals are output to the V/H demodulators 3 and 4, respectively, which demodulate the IF signals into baseband signals. At this time, in the V/H demodulator 3, a correlation between an error signal obtained from one polarization and a demodulation signal of the other polarization-side V/H demodulator 4 is obtained to compensate for cross polarization interference. A similar Operation takes place even in the V/H demodulator 4.
According to the above reception local synchronous method, outputs of the V/H receiver local oscillators 17 and 18 (outputs of VCOs 171 and 181) are each controlled by the operation of the PLL circuits 172 and 182 so as to have the same frequency in synchronization with a common reference signal from the reference circuit 30.
Regarding the above, what is disclosed in PTL 2 is a dual polarization transmission system that uses a transmission local synchronous method as a reference synchronous scheme. The dual polarization transmission system uses a digital demodulator of a semi-synchronous type and an XPIC, and controls the phase of oscillation of a numerically controlled oscillator (NCO) on the basis of a signal from a modulator, thereby tracking a phase difference between other polarizations and eliminating an interference wave.